SCARC Projects and Kits
VK5JST AERIAL ANALYSER
Page information sourced from Jim Tregellas VK5JST's Website. (Thanks Jim!)
This project was published in the May 2005 issue of the Australian magazine "Amateur Radio", and has been designed using parts which are very readily available (see the circuit and parts list). There is little point in supplying a kit of parts to amateurs who have been around a while - most will be in your junk box. However, the printed circuit board (double sided and hot air solder reflow coated) is another matter, and is available here.
Complete Kits AU$135.00 (Australia)
Complete Kits AU$150.00 (International)
Payment can be made by either:
1. Posting Cheque or Money Order to SCARC - PO Box 333 Morphett Vale, SA 5162
2. PayPal
3. Direct Bank Deposit. (contact kits@scarc.org.au for details)
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*OVERSEAS BUYERS*
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*POSTAGE*
An alternate delivery option is available though Australia Post's Express Courier International, which includes track and trace [www.auspost.com/track]. For further details and prices, please email Kits before making payment.
Please Note: due to delayed supply of parts, please contact us on kits@scarc.org.au to determine if kits are available.
DATA
You can Download ALL the files for the "Aerial Analyser", by clicking here (Zipped File).
The code for the PICAXE 28X was prepared using the Revolution Education Programme Editor and is in BASIC format. This will program any PICAXE chip, by Revolution Education.
Note that since submission of the article for publication, a battery voltage monitoring modification has been included at the suggestion of Barry Williams VK5ZBQ. To add this feature to MK1 printed circuit boards, cut the PCB connection between AN3 (microprocessor pin 5) and ground. Then add a 16K resistor between the +12volt rail and AN3, and 3K9 between AN3 and ground. At switch on the battery voltage is displayed for 1.5 seconds.
To obtain the correct voltage indication, simply vary the division ratio in the line "w0=w0/100" in the "battery:" subroutine of the microprocessor code. The divider should lie in the range 94-106, and compensates for the 5% component tolerances. This modication ( and other minor layout and circuit mods ) have been included on the Mk2 version of Analyser detailed below
DOWNLOADS
Further Information
WARTS, PROBLEMS AND COMMENTS
At the time of writing, around 500 units have been built, and apart from the odd dry joint, only 3 units have given serious trouble. For two of these units, the trouble was identical and involved the upper frequency range of the oscillator.
Instead of producing a good sinusoidal output, the generator produces a near sawtooth waveform of about half normal amplitude. This is caused by the 1uH RF choke in the oscillator tank having much lower losses than normal,and consequently causing much higher than normal gain around the oscillator loop. The oscillator consequently "squeggs" producing a weird output, and to overcome the problem, simply reduce the tank Q by paralleling the 1uH RFC with a damping resistor. Both of the troublesome units produced good flat sine outputs when the 1uH choke was paralleled with a 3k3 resistor, but depending on your choke characteristics, it may be necessary to go as low as 1k5. Keep an eye on the waveform purity when you are experimenting.
The remaining unit gave trouble simply because it would not oscillate on the two highest frequency ranges, and had odd looking waveforms on the remaining ranges (flat topped sines etc). This was traced to the use of ordinary standard 100nF disc ceramic bypass capacitors, which being physically large single plate devices, have relatively high losses and low self resonant frequencies.
There is no place in a wide band power circuit (which this instrument is) for such components. Use only miniature multilayer MONOLITHIC bypass (and coupling) capacitors in this circuit with lead lengths of as near to zero as possible, or you WILL end in trouble.
The bypass capacitor at T3 base is particularly important. Minimizing stray capacitance in the test circuit is important, as it appears in parallel with the load and can upset measurement accuracy at high frequencies and high SWRs. It can be minimised by mounting the following components away from the ground plane by 1-2mm. (a) D5 and the associated 100pF and 47K (b) D6 and the associated 100pF and 47K. (c) The 2 X 100 ohm test cct series resistors.
Warning: This kit is of an Advanced standard,and assembly requires good soldering and hand skills. The club takes NO responsibility with problems associated with construction, but will provide reasonable assistance to the purchaser to get the kit operational . However, the purchaser should keep clearly in mind that this is a volunteer organisation with limited resources, and that politeness in correspondence is much appreciated. As the technical article makes clear, access to an oscilloscope is highly desireable but not strictly mandatory.
